.\"	$OpenBSD: tunefs.8,v 1.26 2010/10/18 15:01:40 jmc Exp $
.\"	$NetBSD: tunefs.8,v 1.36 2004/12/20 10:28:47 hubertf Exp $
.\"
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.\"     @(#)tunefs.8	8.3 (Berkeley) 5/3/95
.\"
.Dd $Mdocdate: October 18 2010 $
.Dt TUNEFS 8
.Os
.Sh NAME
.Nm tunefs
.Nd tune up an existing file system
.Sh SYNOPSIS
.Nm
.Bk -words
.Op Fl AFN
.Op Fl e Ar maxbpg
.Op Fl g Ar avgfilesize
.Op Fl h Ar avgfpdir
.Op Fl m Ar minfree
.\" .Op Fl n Ar soft_dependency_enabling
.Op Fl o Ar optimize_preference
.Ar special | filesys
.Ek
.Sh DESCRIPTION
.Nm
is designed to change the dynamic parameters of a file system
which affect the layout policies.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl A
Cause the values to be updated in all the alternate
superblocks instead of just the standard superblock.
If this option is not used,
then use of a backup superblock by
.Xr fsck 8
will lose anything changed by
.Nm .
.Fl A
is ignored when
.Fl N
is specified.
.It Fl e Ar maxbpg
This indicates the maximum number of blocks any single file can
allocate out of a cylinder group before it is forced to begin
allocating blocks from another cylinder group.
Typically this value is set to about one quarter of the total blocks
in a cylinder group.
The intent is to prevent any single file from using up all the
blocks in a single cylinder group,
thus degrading access times for all files subsequently allocated
in that cylinder group.
The effect of this limit is to cause big files to do long seeks
more frequently than if they were allowed to allocate all the blocks
in a cylinder group before seeking elsewhere.
For file systems with exclusively large files,
this parameter should be set higher.
.It Fl F
Indicates that
.Ar special
is a file system image, rather than a device name or file system mount point.
.Ar special
will be accessed
.Sq as-is .
.It Fl g Ar avgfilesize
This specifies the expected average file size.
.It Fl h Ar avgfpdir
This specifies the expected number of files per directory.
.It Fl m Ar minfree
This value specifies the percentage of space held back
from normal users; the minimum free space threshold.
The default value is set during creation of the filesystem; see
.Xr newfs 8 .
This value can be set to zero, however up to a factor of three
in throughput will be lost over the performance obtained at a 5%
threshold.
Note that if the value is raised above the current usage level,
users will be unable to allocate files until enough files have
been deleted to get under the higher threshold.
.It Fl N
Display all the settable options
(after any changes from the tuning options)
but do not cause any of them to be changed.
.It Fl o Ar optimize_preference
The file system can either try to minimize the time spent
allocating blocks, or it can attempt to minimize the space
fragmentation on the disk.
If the value of
.Ar minfree
(see above) is less than 5%,
then the file system should optimize for space to avoid
running out of full sized blocks.
For values of
.Ar minfree
greater than or equal to 5%,
fragmentation is unlikely to be problematical, and
the file system can be optimized for time.
.Pp
.Ar optimize_preference
can be specified as either
.Li space
or
.Li time .
.El
.Sh SEE ALSO
.Xr fs 5 ,
.Xr dumpfs 8 ,
.Xr fsck_ffs 8 ,
.Xr growfs 8 ,
.Xr newfs 8
.Rs
.%A M. McKusick
.%A W. Joy
.%A S. Leffler
.%A R. Fabry
.%T "A Fast File System for UNIX"
.%J "ACM Transactions on Computer Systems 2"
.%N 3
.%P pp 181-197
.%D August 1984
.Re
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.2 .
.Sh BUGS
This program should work on mounted and active file systems.
Because the super-block is not kept in the buffer cache,
the changes will only take effect if the program
is run on unmounted file systems.
To change the root file system, the system must be rebooted
after the file system is tuned.
.Pp
.\" Take this out and a Unix Demon will dog your steps from now until
.\" the time_t's wrap around.
You can tune a file system, but you can't tune a fish.
